Add multi-database discovery and grouping features

This update introduces optional automatic discovery and onboarding of all databases on a PostgreSQL instance. It also enhances the frontend UI with grouped target display and navigation, making it easier to view and manage related databases. Additionally, new backend endpoints and logic ensure seamless integration of these features.

frontend/src/pages/TargetsPage.jsx

@@ -7,11 +7,12 @@ const emptyForm = {
   name: "",
   host: "",
   port: 5432,
-  dbname: "",
+  dbname: "postgres",
   username: "",
   password: "",
   sslmode: "prefer",
   use_pg_stat_statements: true,
+  discover_all_databases: false,
   owner_user_ids: [],
   tags: {},
 };
@@ -276,8 +277,18 @@ export function TargetsPage() {
               <input placeholder="5432" value={form.port} onChange={(e) => setForm({ ...form, port: Number(e.target.value) })} type="number" required />
             </div>
             <div className="field">
-              <label>DB Name</label>
-              <input placeholder="e.g. postgres or appdb" value={form.dbname} onChange={(e) => setForm({ ...form, dbname: e.target.value })} required />
+              <label>{form.discover_all_databases ? "Discovery DB" : "DB Name"}</label>
+              <input
+                placeholder={form.discover_all_databases ? "e.g. postgres" : "e.g. postgres or appdb"}
+                value={form.dbname}
+                onChange={(e) => setForm({ ...form, dbname: e.target.value })}
+                required
+              />
+              <small>
+                {form.discover_all_databases
+                  ? "Connection database used to crawl all available databases on this instance."
+                  : "Single database to monitor for this target."}
+              </small>
             </div>
             <div className="field">
               <label>Username</label>
@@ -313,6 +324,21 @@ export function TargetsPage() {
                 </span>
               </label>
             </div>
+            <div className="field toggle-field">
+              <label>Scope</label>
+              <label className="toggle-check">
+                <input
+                  type="checkbox"
+                  checked={!!form.discover_all_databases}
+                  onChange={(e) => setForm({ ...form, discover_all_databases: e.target.checked })}
+                />
+                <span className="toggle-ui" aria-hidden="true" />
+                <span className="toggle-copy">
+                  <strong>Discover and add all databases</strong>
+                  <small>Requires credentials with access to list databases (typically a superuser).</small>
+                </span>
+              </label>
+            </div>
             <div className="field field-full">
               <label>Responsible Users (Target Owners)</label>
               <OwnerPicker